Abstract: An optical waveguide is formed over a substrate, with an insulating planarization layer formed adjacent the optical waveguide and level with the top of the waveguide A microwave transmission line is formed over the planarization layer and overlies a top surface of the optical waveguide. In this circuit, isolation of a microwave transmission line is provided by forming the line over a planarization layer which fills the space adjacent to an optical waveguide structure. The transmission line can then contact electrically the top surface of the waveguide structure but is isolated from the substrate which supports the waveguide.